In optical networks and other environments, it often is necessary to interface semiconductor optical amplifiers and other semiconductor optical devices to optical fibers, silica planar optical waveguides, and other optical media. These various optical devices and media often have different propagation modes and thus require mode (or spot-size) conversion in order to interface to each other. For instance, optical seminconductor devices such as a semiconductor optical amplifier (SOA) typically have a very small spot size (or mode) compared to an optical fiber or a silica planar optical waveguide. One application for the coupling of an SOA to a silica planar optical waveguide is the construction of a wavelength tunable laser.
The difference in spot-size often is a result of a difference in the refractive index of the light propagating media of the device. For instance, an optical fiber or silica planar optical waveguide typically has a refractive index of about 1.45 and thus, a relatively large mode (or spot size), whereas a semiconductor laser typically has an optical index of about 3.3 and thus a relatively small mode (or spot size).
Several techniques for mode conversion, therefore, are well known and in common use, such as, the use of lenses or mode converters. The use of lenses to mode convert has several drawbacks, including the expense of the optical components and their precise assembly and the need to hermetically package the interface. Another technique for mode conversion is to fabricate an SOA with a horizontal and vertical taper close to its output facet. However, fabricating a vertical taper in a semiconductor is a complex, time consuming and expensive process and often requires an SOA performance trade-off.
With respect to wavelength tunable lasers, one common type is a distributed Bragg reflector (DBR) laser employing grating-assisted couplers and/or sampled gratings. While these lasers have adequate performance, they require complex InP growth and processing, time-consuming testing and calibration, sensitive control, and an external wavelength monitor. They also typically have a small optical mode, requiring precise alignment in order to couple to optical fibers (tolerance of less than 0.1 microns). While such lasers are relatively inexpensive, the above-noted challenges make the price too high for applications such as fiber-to-the-home.
Another common type of tunable laser is the bulk-optic external cavity laser. These lasers also have adequate performance, but require significant hand assembly and have moving parts.
Another, less common type of tunable laser is an array of fixed-wavelength lasers coupled together with a power combiner. The disadvantages of this approach include complicated processing, limited wavelength tuning, and low output power.
Accordingly, one object of the present invention is to provide an improved tunable laser by coupling a standard SOA to a silica planar optical waveguide using an easily fabricated and packaged mode conversion apparatus.
Spot-size conversion in one dimension can be achieved by providing a horizontal taper near the output facet of the semiconductor optical device and orienting it at a 90° angle to the silica planar waveguide layer. Due to the 90° orientation of the semiconductor optical device to the silica planar waveguide layer, the horizontal taper of the semiconductor optical device can result in a matching of the vertical size component of the modes. To match the horizontal size component of the modes we propose to use a high-numerical aperture star coupler as described below.